PROJECT SUMMARY Human MS is a biphasic demyelinating disease: there is an initial relapsing-remitting phase (RRMS) followed by a secondary progressive phase (SPMS). The SPMS is currently incurable. To examine the pathology of SPMS, we studied a monophasic mouse model (Shiverer) that exhibits only the progressive phase. Under support of an R-21 (precursor to this RO1), we made the surprising discovery that deleting SNPH (a major mitochondrial anchoring protein) in Shiverer produces dramatic neuroprotection. In contrast, deleting SNPH offers no protection in another monophasic animal model (EAE/B6) that exhibits only the RRMS phase. We hypothesize that SNPH has a biphasic role in human MS (it is beneficial in RRMS and harmful in SPMS) and that inhibiting SNPH in SPMS is a novel therapy for progressive MS. This grant extends the R-21 results in mechanistic (Aim 1) and translational (Aim 2 & 3) directions. In Mechanistic Aim 1, we will examine how SNPH-KO protects pathologic neurons in vitrro by a two- pronged action to activate neuroprotective mitophagy and mitochondrial fusion pathways. In Translational Aim 2, we will characterize SNPH pathology in a novel biphasic mouse model (EAE/NOD) that is superior to our monophasic models in expressing both RRMS and SPMS phases to better mimic human biphasic MS. The climax of this grant is Translational Aim 3. We will design a treatment for the SPMS phase in the biphasic mouse model as a possible future treatment for human progressive MS. Our treatment design is two-fold. First is timing of therapy. We will use conditional SNPH-KO strategy to show that the best timing for treatment is at the transition from RRMS to SPMS when SNPH stops being beneficial and starts becoming harmful. Second is combinatorial therapy. We will examine whether a combinatorial therapy (SNPH-KO and anti-inflammation) might completely eradicate neurodegeneration in the progressive phase of the biphasic mouse model. Conclusion: This project formulates a brand new framework for designing treatments for the currently incurable progressive MS. Through a biphasic SNPH hypothesis, we pinpoint a key mitochondrial anchoring protein whose function switches from being beneficial to harmful during MS progression. The crux of this grant is to validate a biphasic mouse model to mimic biphasic human MS, then treat the mouse by targeting SNPH in the late phase using conditional and combinatorial strategies as a future step to treat human progressive MS.